In the page builder, any text block has an html editor. You can access the html editor by clicking the <> source button on the text block toolbar.

CSS:

There are several places to add custom css

1. In the brand (Page)
   
   


2. In the page builder (Styles Tab)
   
   


3. On the row (click the hamburger on the row)
   
   

   

   
   

Javascript:

You cannot embed javascript into content blocks, thus there are several places to embed javascript code

1. Globally (Administration->Account Settings->Landing Pages [Global Script Tag]
   
   


2. 
   On the page properties tab
   
   


3. 
   On the confirmation tab in the form editor
   
   


4. 
   On the row of a page (hamburger on the row->click the gear)